In the world of cybersecurity, vulnerabilities are discovered and addressed daily. However, some vulnerabilities can have far-reaching consequences if they are not detected and fixed promptly. One such frailty that has recently come to light is CVE-2021-40438.

This vulnerability has the potential to impact a wide range of systems and has caught the attention of cybersecurity experts worldwide. Cybercriminals can exploit this weakness to access sensitive information or control affected systems.

We will explore what CVE-2021-40438 is, how it was discovered, and the potential impact it could have on affected systems. We will also discuss the available mitigations and remediation steps to help prevent the exploitation of this vulnerability.

What Is CVE-2021-40438?

CVE-2021-40438 is a critical Server Side Request Forgery (SSRF) vulnerability that affects Apache HTTP Server version 2.4.48 and earlier. If a specially crafted request URI path is used, it can trigger mod_proxy to forward the request to an origin server chosen by the remote user.

An SSRF vulnerability enables an attacker to send unauthorised requests from a vulnerable server to another server, often bypassing security controls and potentially leading to various attacks, including data theft, privilege escalation, and even complete system compromise.

Security researchers discovered the vulnerability, and on September 16, 2021, Apache released version 2.4.49 of the HTTP Server, which included a fix for CVE-2021-40438. The severity of the vulnerability was high, and if left unpatched, it could have had a significant impact on affected systems.

A remote attacker can easily exploit this vulnerability to access internal network resources, bypass firewalls, and launch attacks against other systems. In the hands of an attacker, this weakness could lead to devastating consequences, including theft of sensitive data, complete system takeover, and cyber ransomware attacks.

How Was CVE-2021-40438 Discovered?

Security researchers discovered CVE-2021-40438. The security researchers scanned the vulnerability by analysing the code of the Apache HTTP Server, specifically the mod_proxy module.

The mod_proxy module is responsible for proxying a crafted request between the Apache server and other servers, allowing the server to communicate with backend systems. They discovered that the module did not properly validate the scheme of a proxied URL, allowing an attacker to manipulate the URL and potentially make an unauthorised request on behalf of the server.

The vulnerability was reported to the Apache Software Foundation, which acknowledged and confirmed the weakness. Apache then released version 2.4.49 of the HTTP Server, which included security fixes for CVE-2021-40438.

The discovery of CVE-2021-40438 highlights the importance of ongoing security research and testing. Through the efforts of security researchers, vulnerabilities can be identified and addressed before attackers can exploit them.

The Affected Apache HTTP Server Versions

The CVE-2021-40438 security issue affects Apache HTTP Server 2.4.48 and earlier versions. Any system running an affected Apache HTTP Server is vulnerable to exploitation.

It is important to note that while Apache HTTP Server version 2.4.48 is mentioned in the CVE, earlier versions of Apache HTTP Server may also be vulnerable. As such, it is recommended that all users of Apache HTTP Server review their systems to determine whether they are running an affected version.

Users running an affected version of the Apache HTTP Server are strongly advised to upgrade to the latest version, including a fix for the CVE-2021-40438 vulnerability. In addition to upgrading, users should also take additional security measures to protect their systems, such as implementing firewalls, intrusion detection systems, and other security solutions.

Impact of CVE-2021-40438

If exploited, the vulnerability can allow an attacker to perform a wide range of malicious activities, including accessing sensitive information, stealing data, and even taking control of the affected system. The severity of the impact depends on the specific system being attacked and the attacker’s intentions.

Organisations affected by CVE-2021-40438 may experience serious financial and reputational damage and legal consequences, especially if the attacker can steal sensitive data or compromise critical systems. Additionally, downtime due to system compromise or patching can result in lost revenue, productivity, and customer confidence.

Mitigations and Remediation of CVE-2021-40438

The CVE-2021-40438 vulnerability can be mitigated and remediated through a few steps.

First and foremost, upgrading the affected version of the Apache HTTP Server to version 2.4.49, including a vulnerability fix, is crucial. Organisations should prioritise updating their systems immediately to prevent potential exploitation.

Additionally, reviewing and updating security configurations for Apache HTTP Server is recommended to ensure that only necessary services are exposed to the internet. This can be done by reviewing access controls, limiting requests to trusted sources, and implementing other security measures such as firewalls and intrusion detection systems.

It is also essential to conduct a thorough security assessment of the affected system to identify signs of exploitation or unauthorised access. This includes reviewing logs and monitoring network traffic for any suspicious activity.

Finally, organisations should educate their employees on the risks associated with SSRF vulnerabilities and how to identify and report suspicious activity. This includes regular phishing awareness training and providing employees with clear policies and procedures for reporting potential security incidents.


CVE-2021-40438 highlights the importance of staying vigilant against cybersecurity threats. This vulnerability, like many others, can have serious consequences if left unaddressed. Individuals and organisations must take steps to mitigate and remediate vulnerabilities in their systems and adopt best practices for preventing exploitation.

Doing so can help protect ourselves and others from the potentially devastating effects of cyber attacks. Let CVE-2021-40438 remind you of the ongoing need for proactive cybersecurity measures.

Frequently Asked Questions (FAQs) on CVE-2021-40438

1. What should I do if I am using an affected version of Apache HTTP Server?

A: If you are using an affected version of Apache HTTP Server, upgrading to the latest version (2.4.49) is strongly recommended as soon as possible. Additionally, it is important to implement robust security measures such as firewalls, intrusion detection systems, and other security solutions to protect against potential attacks.

2. How can I prevent CVE-2021-40438 from being exploited on my system?

A: The best way to prevent CVE-2021-40438 from being exploited on your system is to ensure that you are running the latest version of Apache HTTP Server (2.4.49).

Additionally, you should implement robust security measures such as firewalls, intrusion detection systems, and other security solutions to protect against potential attacks. Staying up-to-date on the latest security patches and best practices is also important.

3. What are the potential consequences of exploiting CVE-2021-40438?

Exploiting the vulnerability can lead to various attacks, including data theft, server compromise, and unauthorised access to sensitive information. Additionally, an attacker may use the vulnerability as a stepping stone to launch more sophisticated attacks against the targeted organisation.

4. Has a fix been released for CVE-2021-40438?

Yes, Apache released version 2.4.49 of the HTTP Server on September 16, 2021, which includes a fix for CVE-2021-40438. Users of affected versions of Apache HTTP Server are strongly recommended to upgrade to the latest version to mitigate the vulnerability.

5. What version of Apache is CVE 2021 40438?”

CVE-2021-40438 is a vulnerability that affects Apache HTTP Server version 2.4.48 and earlier. If you are running any of these versions, it is important to update to the latest version as soon as possible to ensure your system is protected against this vulnerability.

6. What is the CVSS score of CVE 2021 40438?

The CVSS score of CVE-2021-40438 is 9.8 out of 10, which is considered a critical vulnerability. This score indicates that the vulnerability has a high impact and is easily exploitable, which makes it a significant threat to affected systems.

Organisations must address this weakness promptly by applying the necessary patches or workarounds to protect their systems from attacks.

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